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  • Writer's pictureWatlington Climate Action Group

A window is a radiator

It's hot in the UK this week. Not hot like a north American heat dome, but still uncomfortably warm, and hot enough that the Met Office has issued its first 'extreme heat warning' -

Are you managing to keep your house bearable? Is it overheating?

Research published in Buildings & Cities Journal reported that in the UK's hottest summer (2018), 15% of homes (3.6 million) overheated. The prevalence of overheating is likely to increase as climate change delivers us hotter summers (see links), that's a LOT of overheating homes.

This includes my house: a fairly conventional 1970s detached. The front faces southwest and is unshaded, and the heat builds up continually from lunchtime to sunset.

We're getting bedroom temperatures up to 28°C at the moment, and struggling to purge the heat at night.

Why? Aside from the obvious ', it's hot outside, our windows are pretty old, and any formerly useful insulation properties of the double-glazing are long gone (we're currently in the process of replacing them with lovely new Green Building Store windows.

Here's a quick building physics lesson (with apologies to all of you with even a basic understanding of the subject!)

Heat in buildings is transferred by three mechanisms (detailed explanation in links):

- Conduction

- Convection

- Radiation

And a window is perfect to facilitate all three of these methods of heat transfer.

It's no coincidence that the co-creator of Passivhaus, Wolfgang Feist, dubbed windows the 'radiators of a Passivhaus'. When a building's heating energy demand is radically reduced by a high-performing fabric, some of the remaining heating demand is met by what are known as 'beneficial solar gains' from windows. But these are only 'useful' in wintertime, to offset heating energy demand. In summer, gains from windows aren't so useful. Passivhaus, of course, is carefully designed to create the ideal balance of heat gains and overheating mitigation.

Anyone who's designed and then built a south-facing conservatory or huge expanse of windows (and not properly mitigated the summer solar gains) will know that that room overheats in summer!

So, what to do?

- CLOSE your windows when the sun is on them - this stops the heat directly flowing in

- CLOSE the blinds or curtains - this reduces the radiative heat gain from the window getting into the room (and stops the sunshine beaming in!)

- OPEN them all once the sun is off the windows in the early evening

- PURGE ventilate if that's possible (open windows and doors upstairs and downstairs on opposite sides)

- DESIGN windows - especially large expanses - carefully, to avoid unwanted summertime solar gains and overheating.

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